Midyear Engine Comparison

[Kevin]

I am in the process of purchasing a '64 327/300hp 4-spd. coupe. I would like a little more performance. I want to emulate the 350hp engine of '65-'70, replacing the current 3.08 gears with 3.36's and making a few engine mods. to the bone-stock 300hp engine. Nothing that can't be easily turned back to original, however. I would like to replace the 300hp cam with a 350hp cam, replace the 300hp intake manifold with a 350hp unit, and replace the dual-snorkel aircleaner with an open element unit. My question is with respect to the cylinder heads. The 300hp and 350hp cylinder heads on a '66 (for example) carry the same casting #'s. Does this mean that they are the same configuration (i.e. chamber cc size, valve sizes, ports, etc.)? I believe the 300hp had approx. 10.25-1 comp., while the 350hp was at 11.0-1. Was this due to differences in the head or perhaps in the pistons? To make a long story short, would the mods. I have mentioned above also require changes to the cylinder heads (or different heads)?

Thanks, Kevin

[Duke W.]

Re: Midyear Engine Comparison

The heads are the same casting, but the L-79 finished version is machined for the 2.02/1.60 inch valves vs. 1.94/1.50 for the L-75 head. The L-79 used the domed forged pistons to achieve the higher CR. You could simply install the L-79 cam in your engine and get most of the benefits, and you really don't want the higher CR unless you want to go through the hassle of buying leaded racing gasoline to make you own blend with pump premium to keep the engine out of detonation.

I'd run your production manifold and carb. That way the engine will retain its production visual appearance. The only giveaway will be a different idle quality. The L-79 used an aluminum manifold and Holley carb rather than a cast iron manifold and AFB. It will be an expensive swap, because these original parts are becoming rarer, but not really gain much power. Most of the power differnce between the L-75 and L-79 is in the cam.

Duke

Duke

[Bill Clupper]

Re: Midyear Engine Comparison

I'd have to agree with Duke, I've personally done the job he discusses on a car with a 3.36 gear and it worked well. If you really wnat to help the car, try the cam change and go to a 3.70 gear you will really notice the difference and still look like "nothing unusual". High compression and the attendent gas problems are a pain. I'd recommend pulling the heads and going to hardened valve seats, the change in valve sizes is optional, the extra $ is hard to see in an engine running through mufflers.

[Joe L.]

Re: Midyear Engine Comparison

Kevin----

Well, I guess it all depends how far that you want to go with this with respect to achieving 350 hp (L-79) configuration.

First off, your cylinder heads should be GM #3782461. These are, nominally,62 cc combustion chamber heads. For your L-75, they were equipped with 1.94/1.50 valve size. For 1965 L-79 applications, these same exact heads were fitted with 2.02/1.60 valve size. So, you can have your heads modified for the larger valves if you wish and you will have L-79 heads. This change will be irreversible, but no one will ever know the difference if you decide to return the engine to l-75 configuration at some point.

You will also need to change pistons. Your L-75 pistons are cast aluminum and produce a compression ratio of about 10.5:1. The L-79 used forged pistons producing 11.0:1 compression. These pistons are GM-discontinued but available from Federal Mogul-TRW.

Your connecting rods are the same, so these won't need to be changed. I'd recommend re-sizing though.

As far as the crankshaft goes, both the L-75 and L-79 engines use a forged crankshaft of the same forging number. However, my recollection is that the L-75 does not have the snout drilled and tapped for a balancer retaining bolt. Since the forgings are the same, you may be able to have yours so-modified. This will be advisable if you are to use the 8" harmonic balancer.

Which brings us to the balancer. Your engine uses a 6-1/2" or so balancer. L-79s use an 8" balancer designed for the higher RPM which the L-79 achieves. Although slightly different in configuration than original L-79 balancers(which had reinforcing ribs on the inside of the inner hub) a balancer is still available from GM for L-79 engines. This balancer is GM #3817173.

For the camshaft, your L-75 uses GM #3896929. The L-79 engine uses GM #3863151. The latter camshaft is still available from GM. Otherwise, exact reproductions are available from Crane and others. If using the GM cam, I recommend using NEW stock GM hydraulic lifters of GM #5232720.

For an intake manifold, your L-75 uses a cast iron manifold of GM casting number 3799349. L-79 applications use an aluminum manifold of casting number 3844461. This manifold, last available under GM part #3893594, is long-since discontinued. So, you will need to find a used one. These manifolds sell for in the $300-500 price range.

You could use your existing exhaust manifolds if they're original. Your 64 L-75 was originally equipped with the same 2.5" outlet manifolds as used for SHP engines, including the L-79.

All L-79s, except 1968, used a Holley carburetor. Original, reproduction, or Holley aftermarket carburetors of equivalent specifications are available. The 1966 L-79 was a Holley 3367A and the 1967 used Holley #3810A, if you're interested.

You can use your existing distributor recurved to L-79 specs and equipped with vacuum control GM #1116236 (GM-discontinued, but available in the aftermarket from Standard Auto Parts or Filko).

Oh, by the way, depending upon how far you go with your "conversion" to L-79 specs, this IS NOT going to be an inexpensive project. But, you've probably guessed that by now.

[Kevin]

Re: Midyear Engine Comparison

Thank you for the responses. It is not my intent to "exact" the 350hp, but only to "borrow" a few performance enhancements from it, in order to increase the performance of the 300hp. I thought that the 350hp had 2.02's and the 300hp had the 1.90's, but I was not certain. This modification to the stock 300hp heads would be inconspicuous, but expensive, unless I were in the middle of an engine rebuild anyway (which I do NOT intend to do). I have no intention of changing pistons to achieve a higher CR. I like the idea of the 3.70 gears, however the car is equipped with AC (Vintage, not factory) and I do not want to burn out the compressor. Therefore, I think a cam change (to the 350hp cam) and 3.36 gears is going to be my limit. Should the 350hp cam work acceptably with the 300hp heads (i.e. 1.90 valves) and 300hp intake and CR?

Thanks again, Kevin Morris

[Jeff]

IMHO, not worth the effort in that configuration.

If that's all you really want to do to the motor, I'd say it's not worth going to all that trouble. I certainly wouldn't swap gear sets for just a few points. If you do make any changes, I'd make them one at a time and see if you are where you want to be. I'd start with a gear change to 3.70s or possibly even 4.11s and see how that feels to you. If you do decide you want to mess with the induction, I'd use new aftermarket stuff like cam, lifters, manifold and carb for cost considerations and to know the condition of what you're getting. To me, the only thing really worthwhile would be a gear change so I'd start there and have the engine tuned by someone who knows what they're doing (do you live near Duke, Joe or Dale?). Try that for a while and see if you feel you need to do more.

JP

[Joe L.]

Re: Midyear Engine Comparison

Kevin----

Yes, the 350 horsepower cam will work just fine with your L-75 heads, 10.5:1 compression ratio, and existing Carter AFB carb.

As a matter of fact, this is exactly what I'd recommend that you limit your mods to as others have also recommended. But, you originally asked how you could convert your engine to L-79 (350 hp) specs, so that's why I provided the previous information.

[Kevin Morris]

Re: Midyear Engine Comparison

Joe,

Sorry, I did not make myself very clear the first time around. The info. you provided was very complete. Now, knowing for certain what all of the differences are, I have a solution.

Kevin Morris

[Duke W.]

Cast or forged crank

Joe - It's always been my understanding the the 300 hp engine used a cast crank while all L-79s were nitride hardened forged steel. Also, I recall some discussion on the Board stating that the later L-79s, beginning in '66, I recall, reverted to the two inch manifolds.

Duke

[Joe L.]

Re: Cast or forged crank

Duke-----

Taking your questions in reverse order, you are correct on the exhaust manifolds. 1965 was the last year for the 2-1/2" outlet manifolds on ANY small block engine. Beginning in 1966, ALL small blocks used 2" outlet manifolds. However, for 1964 and 1965, L-75 300hp engines with manual trans did use the 2.5" outlets which were also used, of course, on 63-65 340, 350, 360, 365, and 375 hp engines.

As far as crankshafts go, ALL 1962 through 1967 327 cid engines used forged steel crankshafts, although special surface treatments like nitriding or parkerizing were reserved for special high performance engines. Also, all 1962-67 small block crankshafts(except 1967 350 cid) used the small journal main rod sizes of 2.30/2.00. In 1968, most 327 engines started receiving nodular cast iron cranks. However, these cranks are not easily retrofitable to earlier 327s (or vice-versa) since all of these used the large main/rod journal sizes of 2.45/2.10. The only exception to the cast nodular cranks for 327s in 1968 were the L-79 engines used in Corvette, Chevelles, and Novas as well as certain 1968 heavy duty truck engines. All of these used the large journal sizes and are among the rarest of Chevrolet crankshafts. Forged crankshafts were not used for any 327 engines during 1969, the last year of 327 engine production.

That's the story.

[Bill Clupper]

Re: IMHO, not worth the effort in that configurati

Jeff may have hit the nail on the head, a well tuned 300 hp is nothing to sneeze at, then again, there's always room for a little more...

[Bob Mitchell]

Re: Midyear Engine Comparison

I'm confused, it's my understanding that all "461" heads have a valve size of 1.94/1.50, and the 2.02/1.64 valve size was a modification. All the reading I,ve done about these heads including the "X" head identify the smaller valves. Any thoughts?

Bob

[Joe L.]

Re: Midyear Engine Comparison

Bob----

The 3782461X cylinder heads were originally only produced in the 1.94/1.50 valve size. These heads were produced during the 1961-63 model years.

The 3782461 cylinder heads produced during the 1964-66 model years were originally machined for BOTH 1.94/1.50 AND 2.02/1.60 valve size.

3982461X or 3782461 cylinder heads originally manufactured with 1.94/1.50 valve size can be easily modified for 2.02/1.60 valves and I'm sure that many have been so-modified.